The present invention relates to a switching device for linking various electrical voltage levels in a motor vehicle, in which a drive voltage level has an electric drive machine which may be actuated by a power converter, and a drive energy accumulator which is associated with an intermediate circuit, and in which the drive voltage level is connected to a vehicle electrical system voltage level by means of an electrical converter.
In automotive manufacturing, electrical machines are used for electric and hybrid vehicles. On account of their good control capability and performance, rotary current drives having different numbers of phases and pairs of poles in both asynchronous and synchronous designs are finding increasing acceptance as drives. From a design standpoint, the aim is to provide the highest possible voltage (200-300 V) to the drive in order to reduce power loss to a minimum at a high power density. The voltage level of 12V typically used in the vehicle, or 42 V planned in the future, cannot be advantageously used for a high-power drive. Therefore, coupling of the voltage levels of a high-power drive and a vehicle electrical system requires additional adaptive devices. In particular, when the high-power drive (generator function) is intended to assume the function of supplying the vehicle electrical system, implementation of such adaptive elements is very complicated.
It is known from the general prior art, for example from DE 103 13 752 A1, to connect various voltage levels in multivoltage vehicle electrical systems in motor vehicles by means of a (bidirectional) DC/DC converter. In particular, for hybrid and electric vehicles it is known to provide a DC/DC converter between a drive energy accumulator or an intermediate circuit and the vehicle electrical system.
A disadvantage of the known technology is that it results in great system complexity, high system costs, large installation space requirements, and added weight. For example, for a vehicle electrical system power output of 4 kW, a DC/DC converter requires an installation space of least 5 liters and weighs more than 10 kg. Further expenses result from additional devices, such as cooling and monitoring, which are necessary for reliable operation of the DC/DC converter.
An object of the present invention, therefore, is to provide a switching device for linking various voltage levels in a motor vehicle, which for at least the same functionality as a DC/DC converter is simpler and more economical.
This and other objects and advantages are achieved in accordance with exemplary embodiments of the present invention in which the electrical converter is designed as a coupling circuit, which on the drive side is connected to at least one node point of a winding circuit of the electrical drive machine and to a voltage potential relative to the intermediate circuit, at the vehicle electrical system side the coupling circuit being connected to the vehicle electrical system via a switching unit which has at least one non-diminishing, finite impedance.
According to the invention, the electrical machine itself is used for linking the voltage levels of the electrical machine and the vehicle electrical system. The vehicle electrical system is directly coupled to the electrical machine via the coupling circuit. This circuit makes use of the fact that the potential of the node point may be adjusted by actuating the power converter, at least in a stepwise manner, between the potentials of the intermediate circuit. The frequency of the node point voltage relative to the intermediate circuit is independent of the frequency of the resulting field at the electrical machine to be controlled.
By means of this additional degree of freedom it is possible to adjust the power flow in the coupling circuit in a wide range, independent of the mechanical power output of the electrical machine. The adjustment capability is limited only by the effective impedance of the original circuit and the additional circuit. The coupling circuit may be designed for monodirectional or bidirectional power flow; i.e., the intermediate circuit (including the connected drive energy accumulator) may be supplied from the vehicle electrical system, and the vehicle electrical system may be supplied from the intermediate circuit. The coupling circuit is operable in any working range of the electrical machine. In particular, the direction of the power flow within the coupling circuit is not dependent on the working quadrant (magnetization curve) of the electrical machine.
The complexity of the coupling of the various voltage levels is minimized by use of the switching device according to the invention. In particular, this means no additional magnetic elements, fewer semiconductor components, fewer actuating devices, less cost for the connection technology, and lower design costs for cooling and a housing. The additional elements may be integrated into the power converter or power inverter portion of the drive, using the technologies already employed therein. Reduction of the complexity results in lowered costs associated with the system, the installation space, and the weight. Compared to a classical DC/DC converter of the same functionality connected between the voltage levels, the outlays for complexity, installation space, weight, and costs are reduced by more than two-thirds.
The coupling circuit according to the invention may be connected in a particularly simple manner in the form of a bidirectional converter. Additional functionality is achieved in comparison to conventional circuits using DC/DC converters, in that the drive energy accumulator, having higher voltage compared to a vehicle electrical system or vehicle electrical system energy accumulator (vehicle battery), does not have to be supplied from the vehicle electrical system when the vehicle is not in use. This minimizes storage losses from the drive energy accumulator. In particular, the drive energy accumulator may actively discharge into the vehicle electrical system when the vehicle is shut off, and upon start-up of the vehicle may be recharged in a very short time.
A further advantage is that the electrical machine may also be supplied with drive power directly from the vehicle electrical system via appropriate control, or may also act directly as a generator power converter between the mechanical output and the vehicle electrical system. The efficiency chain of the energy conversion may be shortened compared to a conventional circuit using a DC/DC converter, with the result that the overall system has a correspondingly higher efficiency for the conversion between electrical energy and mechanical energy. Due to the possibility of direct power conversion between the vehicle electrical system and the mechanical output, this may also provide emergency operation in the event of a defective energy accumulator in the higher-voltage level, or for a (partially) defective current converter or inverter. Both a starter and a generator function may be provided by the direct coupling of the vehicle electrical system and the electrical machine.
According to one exemplary embodiment of the invention, the electrical drive machine is designed as a multiphase rotary current machine which is connected to a winding circuit designed as a star connection, and which may be actuated by a power converter designed as a current inverter, and the coupling circuit is connected on the drive side to at least one node point designed as a neutral point in the star connection of the rotary current machine.
The coupling circuit may be connected to a star connection in a particularly efficient manner. The vehicle electrical system is connected via the coupling circuit to the neutral point (node point) of the multiphase machine connected in the star, and is also connected to the potential relative to the intermediate circuit or the energy accumulator. The potential of the neutral point may be adjusted between the potentials of the intermediate circuit by actuating the current inverter (converter).
In principle, the coupling circuit according to the invention is suited not only for a “classical” three-phase rotary current machine or a rotary current machine having more than three phases, but may also be used for a single-phase brushless direct current machine, i.e., a single-phase motor. It is important for the machine to allow a winding tap for a motor coil.
According to another exemplary embodiment of the invention, the voltage potential relative to the intermediate circuit is specified by the midpoint of an intermediate circuit voltage. In this manner the coupling device for the vehicle electrical system as well as the electrical machine may be operated in electrical symmetry, once again resulting in simpler regulation and control designs than for a configuration directly at a coupling voltage related to an intermediate circuit potential.
According to another exemplary embodiment of the invention, a voltage limiting unit is provided, by use of which overvoltages occurring at one or more node points of the winding circuit may be limited. The voltage limiting unit may be designed as electrical clamping elements. By clamping the node point or neutral point with respect to the intermediate circuit, overvoltages created by a controller for the vehicle electrical system voltage may be limited at the neutral point. Additional clamping elements, for example clamping diodes, may be omitted when appropriate use is made of the magnetic flux coupling in the electrical machine, or the individual inductances in a flux equilibrium are controlled. Clamping may also be performed by capacitive discharge of the neutral point.
According to another exemplary embodiment of the invention, for an electrical machine having multiple node points associated with the star circuit, the coupling circuit is situated between the node points. In this manner, during advantageous control, the intermediate circuit is not subjected to stress from the ripple current (charging current and discharging current) which occurs.
According to another exemplary embodiment of the invention, control of the coupling of the voltage levels is integrated into a control unit for the electrical machine. This allows good control capability, even in the load limit range of the system, with relatively low system complexity.
According to a further exemplary embodiment of the invention, the vehicle electrical system and the winding circuit are coupled via an isolating transformer. This allows an energy exchange between the circuit areas with a secure electrical (galvanic) separation. A further advantage is that the capacitive load on the neutral point is minimized, and the current received by the vehicle electrical system still appears in converted form in the drive energy accumulator, current inverter, and electrical machine. This allows the design of individual components of the switching device to be optimized.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.